Block diagram of the components of the sea rescue system. The responsibilities of Fraunhofer FHR are highlighted.

Conventional navigation radar systems are normally not capable of detecting small objects on a rough sea surface. A modular expansion of existing radar systems and the utilization of cost-effective transponders should remedy this situation.

With increasing swell, it is very difficult or even impossible to detect small drifting objects on the sea surface, e.g. water sports enthusiasts, life rafts or castaways, using conventional maritime radar systems. The radar reflections of these objects are weaker or only slightly stronger than the reflections from the uneven water surface (sea clutter). Research into an innovative sea rescue system, which uses radar to detect people and rescue devices in the water, is currently being conducted within the framework of the publicly-funded joint project SEERAD (Sea rescue system based on low-interference radar).

Here, activities focus on the development of compact and cost-effective transponders that return a frequency-doubled radar signal that is received and evaluated by a harmonic – and also yet to be developed – radar system. This signal is not superimposed by typical interference caused by reflections on the waves with the result that all objects with transponders can be visualized.

The possibility of integrating the harmonic radar system with existing S-band navigation systems of the industrial partner is an important aspect of the project. The conventional navigation radar and the harmonic radar will share a common antenna and, for this reason, the transmit frequency range of both systems must correspond within the S-band. The harmonic radar will receive the frequency-doubled scattering signals of the transponder at the doubled frequency in C-band, while the conventional radar continues to receive the linear reflection signals in the S-band. Due to the utilization of a common radar antenna, both systems have to use the same transmit polarization. Navigation radar systems typically use horizontal polarization. This, however, is not very suitable for the detection of objects that only slightly protrude from the water surface. For this reason, vertical polarization was selected as the receive polarization of the sea rescue radar.

As shown in the block diagram in Fig. 2, Fraunhofer FHR will focus on the investigation of innovative antenna structures for the transponder and the expanded navigation radar. Work on the transponder antenna, which is now largely completed, led to the development of a transponder with dimensions similar to those of a credit card. The integratability of the transponder in conventional life jackets was successfully tested in simulations and experiments conducted in dry and wet environments. As shown in Fig. 3, detectability is investigated from all spatial directions, including possible shading effects.

Maritime navigation systems usually use mechanically rotating fan beam antennas based on slotted rectangular waveguides. In the S-band, these antennas have a length of approximately four meters and have to contend with the environmental conditions at high sea. Fraunhofer FHR faces the special challenge of developing a compact antenna which meets the same high frequency and mechanical requirements as a conventional maritime radar antenna but can also receive with a different polarization at the doubled transmit frequency. A shorter demonstrator antenna has already been successfully constructed. The suitability of the selected approach was verified by measurements carried out in one of FHR’s antenna measurement chambers and operational tests using an initial demonstrator of the harmonic radar hardware.

The next step involves the construction of a combined navigation and sea rescue antenna in full length so that the narrow beamwidth of the antenna in the horizontal plane, as required in the international shipping regulations, can be achieved. After completion and characterization, the antenna will be made available to the industrial partner for extensive measurement campaigns on the Baltic Sea.

The project is funded within the context of the BMBF initiative »Innovative Rescue and Safety Systems« as part of the »Research for Civil Security« program of the Federal Government. The project is led by the University of Applied Sciences in Aachen. The industrial partner is Raytheon Anschütz GmbH. The professional association Seenot-Rettungsmittel e.V. is an associated partner.